An ethylene/TFE or CTFE copolymer (hereinafter referred to as E(C)TFE) is excellent in chemical and thermal resistance, and electrical properties. Further, since it is melt-moldable by injection or extrusion molding, it is widely used in various fields as a material for various kinds of molded articles, a coating of an electrical wire or cable, a lining and a coating.
In order to improve its mechanical properties, especially at a high temperature, it is proposed to copolymerize a third monomer as a modifier with E(C)TFE. For example, U.S. Pat. No. 3,624,250 and Japanese Patent Publication No. 23671/1972 disclose copolymerization of a vinyl monomer having no telogenic activity and providing the copolymer with side chains having at least two carbon atoms as the third monomer with E(C)TFE. Examples of such the vinyl monomer are vinyl compounds of the formulas: EQU R--CF.dbd.CF.sub.2, R.sup.1 O--CF.dbd.CF.sub.2, R.sup.2 CH.sub.2 CX.dbd.CH.sub.2 and R.sup.3 OCH.sub.2 CX.dbd.CH.sub.2
wherein R and R.sup.1 are each a cyclic or acyclic C.sub.2 -C.sub.8 hydrocarbon group including an aromatic group, R.sup.2 and R.sup.3 are each a C.sub.1 -C.sub.7 perfluoro- or chlorofluoroalkyl group and X is hydrogen or methyl, and pentafluoro(2-methylene-4-methyl-1,3-dioxolan.
In addition, U.S. Pat. Nos. 4,123,602 and 3,847,881 disclose CH.sub.2 .dbd.CHC.sub.n F.sub.2n+1 wherein n is an integer of 2 to 10 and CH.sub.2 .dbd.C(CF.sub.3).sub.2 respectively as the monomers copolymerizable with E(C)TFE.
Although its mechanical properties at a high temperature are improved effectively, the E(C)TFE modified with these third comonomers has, however, drawbacks such as follows:
The monomer having a perfluorovinyl group (eg. the monomers of the formulas: R--CF.dbd.CF.sub.2 and R.sup.1 O--CF.dbd.CF.sub.2) has low reactivity with ethylene, TFE and CTFE so that it should be used in a larger amount than the stoichiometric amount to obtain a copolymer having a predetermined monomeric composition and the reaction rate is greatly decreased. Further, such monomer is not commercially advantageous since it is expensive and not easily available.
When the monomer having a tertiary hydrogen such as ##STR1## in the molecule (eg. R.sup.2 CH.sub.2 CH.dbd.CH.sub.2, R.sup.3 OCH.sub.2 CH.dbd.CH.sub.2 and CH.sub.2 .dbd.CHC.sub.n F.sub.2n+1) is used as the modifier, the thermal resistance of the resulting copolymer is deteriorated and it tends to be easily colored and deteriorated at a high temperature. For example, when a copolymer obtained by copolymerizing such monomer with ethylene and TFE at 15.degree. C. and having a melting point of 270.degree. C. is heated at 350.degree. C. for 10, 20 or 30 minutes, color is developed from yellow to brown and the polymer is partly carbonized. Practically, the color development of the copolymer may be a problem when it is molded or, for example, pelletized at such a high temperature.
As a result of an extensive study to overcome the drawbacks of the conventional E(C)TFE, it has now been found that the copolymerization of a certain specific fluorovinyl compound as the third monomer with ethylene and TFE or CTFE improves the thermal resistance and the stress crack resistance of E(C)TFE at a high temperature without deteriorating its inherent properties, and that its copolymerizability as the third monomer is better than the conventional vinyl compounds.